I've ordered some of the 55-32's - slightly cheaper from rapid (using my education account) as I needed a few other bits & pieces from there anyway - they have a lead time of 5-7 days - which is ok for me as I'm back to work tomorrow & probably need to dedicate a few weeks attention away from my project & towards work & my own family and at the moment

Also don't forget the reversed biased diode across the coil of the relay. This wasn't shown in the schematic. Standard practice is to locate the diode as close to the relay coil as possible. The ULN chip has these built in, but they are located on the chip, and I wouldn't depend on them......

I've ordered some of the 55-32's - slightly cheaper from rapid (using my education account) as I needed a few other bits & pieces from there anyway - they have a lead time of 5-7 days - which is ok for me as I'm back to work tomorrow & probably need to dedicate a few weeks attention away from my project & towards work & my own family and at the moment

Did you also buy the required relay socket to connect the wires to this relay?

Also don't forget the reversed biased diode across the coil of the relay. This wasn't shown in the schematic. Standard practice is to locate the diode as close to the relay coil as possible. The ULN chip has these built in, but they are located on the chip, and I wouldn't depend on them......

I've ordered some of the 55-32's - slightly cheaper from rapid (using my education account) as I needed a few other bits & pieces from there anyway - they have a lead time of 5-7 days - which is ok for me as I'm back to work tomorrow & probably need to dedicate a few weeks attention away from my project & towards work & my own family and at the moment

I'll let you know how I get on with it.

Just wanted to let you know that it all works

...well at least my testing of it all does - I can now turn a mains powered light on/off at will via an mcp23017 connected to a ULN2803A connected to the 55.32.9.012.0040 relay with a separate 12V power supply.
(@pcmanbob - your circuit diagram was very useful when returning to this after several weeks)

...so I assume I'll be able to turn on/off my smoke machine & lights etc for my project (have a week off work soon, so I'll be back to it!)

I have determined that whenever the mains AC line is connected to the output, there is approximately 2V across the GND-In(any) of the relay inputs - this is present even if there are no other connections to the relay board.

Update

Sorry I did not read your last post saying that you already solved the problem using ULN2803. So my guess of the faulty relay is wrong. Don't bother to read the following paragraphs. I should read your old posts to see how you solved the problem.

200V+AC Mains Leaking 2V (AC or DC?) to the Input of 5V Relay

Introducing myselfIn the last 6 months I have been playing with various 5V Relays (High trigger, Low trigger, direct or opto isolated, switching 12VAC/DC, 24VAC, 220VAC DC or inductive loads), using Rpi GPIO, either 3V direct, or shifted 5V. I also played with MCP23017. You may like to check out some of my old posts:

QuestionHave you used a scope to check that the 2V "noise" is DC or AC? Usually AC noise is of the order of 100mV peak to peak. 2V is a huge leak!

My wild guessMy first guess is that your relay board is leaking AC signal. My first troubleshooting step is replace another relay and check again. Your relay is optoisolated, if there is luckily also a JD-Vcc jumper, it might help disconnecting the relay power ground from the Rpi PSU ground, for total optical isolation.